1. Field of the Invention
The invention relates to the production of a metal silicon and somewhat more particularly to a reaction container for deposition of elemental silicon onto heated silicon mandrels positioned within such a container from a reaction gas fed to the container which is capable of thermal decomposition to yield elemental silicon.
2. Prior Art
U.S. Pat. No. 3,918,396, for example, describes a reaction container for deposition of elemental silicon onto heated mandrels positioned within the container. Generally, such a container comprises a plate-shaped support member comprised of an upper silver plate and a somewhat larger lower steel plate. The support member is provided with suitable openings for mounting thereon in a gas impermeable manner spaced apart and appropriately insulated electrodes so that one end of such electrodes extends above the silver plate and the other end extends below the steel plate for connection to a voltage source. The support member also includes a suitable opening for mounting thereon in a gas impermeable manner gas inlet and outlet tubes, which may be concentrically arranged so that the inner tube comprises the gas inlet and the other tube comprises the gas outlet. A pair of elongated mandrels, preferably composed of pure silicon, are vertically mounted on the electrode ends above the silver plate and connected to one another at their upper ends with a bridge member composed of a temperature-resistant electrically conductive material. The quartz bell jar, which may include a flange along the lower edge thereof, is positioned in a gas impermeable manner on the silver plate and out of contact with the steel plate. This combination of bell jar and silver plate define the reaction container, which in the embodiment described in U.S. Pat. No. 3,918,396 is surrounded by a pressure chamber containing a pressurized gas forcing the lower edge of the bell jar against the silver plate in a gas impermeable manner. The pressure chamber is defined by a steel or the like container adapted to be positioned onto the steel plate of the earlier described support member and includes a gas inlet for the pressurized gas and a monometer for controlling the pressure within the chamber of the steel container. A pressure-resistant observation window may be provided along a wall portion of the pressure container for observing the deposition of silicon on the mandrels within the bell jar.
When this type of apparatus is utilized for depositing large amounts of silicon, difficulties are encountered in that the first deposited silicon is purer than the later deposited material. Apparently, this is due to increasing thermal stresses on the quartz or rotosil bell jar.
Of course, this drawback is also present when more conventional reaction containers are used, i.e., the reaction apparatus as described in U.S. Pat. No. 3,918,396 but without an external pressure container.
The thermal stresses on the walls of a reaction container cannot be easily compensated by a corresponding cooling because this would endanger the deposition process occuring within the reaction container.